The present invention relates to a developing device applicable to a copier, facsimile transceiver, printer or similar image forming apparatus and using a single component type developer, i.e., a toner. More particularly, the present invention is concerned with a developing device having a developer carrier capable of forming numerous microfields thereon.
With an image forming apparatus of the type forming a latent image electrostatically on an image carrier and developing it by a developer, it is advantageous to use a developing device operable with a single component type developer, i.e., a toner from the size, cost and reliability standpoint. Particularly, a developing device using a nonmagnetic toner is advantageously applicable to a color image forming apparatus since such a toner is extremely clear. A conventional developing device operable with the toner has a developer carrier for transporting the toner along a predetermined circulation path including a developing region, storing means for storing the toner, and developer supplying means for supplying the toner to the developer carrier, as disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 42672/1986 and 238072/1986.
In a developing system using a nonmagnetic toner, for example, an optimum amount of toner deposition on the developer carrier and an optimum charge to be deposited on the toner are as follows. Preferably, the toner should be deposited on the developer carrier such that the amount of toner is about 0.6 mg/cm.sup.2 to 1.0 mg/cm.sup.2 as measured on the developer carrier or about 0.5 mg/cm.sup.2 to 0.7 mg/cm.sup.2 as measured on a recording medium. The amounts of toner deposition on the image carrier and recording medium are effected not only by the amount of toner deposited on the developer carrier, but also by the relative speed of the image carrier and developer carrier as measured in the developing region.
However, the problem with this type of conventional developing device is that the toner is deposited only in a single layer on the developer carrier. Hence, although the toner transported to the developing region carries a mean charge of 5 .mu.c/g to 15 .mu.c/g, the amount of toner deposition on the developer carrier is as small as 0.2 mg/cm.sup.2 to 0.8 mg/cm.sup.2. It follows that the desired amount of toner deposition on, for example, the image carrier is not achievable unless the developer carrier is moved at a speed two to four times as high as the speed of the image carrier.
Assume that the rotation speed of the developer carrier is increased to compensate for the short toner deposition on the developer carrier. Then, it is difficult to increase the image forming speed. Moreover, when a solid image is reproduced, the density becomes higher at the trailing edge portion of the image than at the other portion. This occurrence does not matter in the case of a black-and-white image. However, in the case of a color image, the density increases at the trailing edge portion of the image since the color is recognized through the toner. Particularly, when a plurality of color components are combined to form a composite color image, the resulting colors will appear different from expected ones.
To eliminate the above-mentioned local increase in image density and deposit a desired amount of toner on, for example, the image carrier, it is necessary to bring the speed of the developer carrier close to that of the image carrier, i.e., to execute substantially equispeed development. At the same time, it is necessary to deposit a greater amount of toner on the developer carrier than conventional. Specifically, in order that the toner may be deposited in a sufficient amount on the image carrier and recording medium by the equispeed development, the prerequisite is that the toner be deposited on the developer carrier in an amount of at least 0.8 mg/cm.sup.2 for contact development which is efficient or in an amount of at least 1.0 mg/cm.sup.2 for noncontact development which is less efficient. This in turn requires the toner to form two or more layers on the developer carrier. Moreover, should uncharged toner particles and inversely charged toner particles exist in the toner layer on the developer carrier, they would obstruct the transfer of the toner, contaminate the background of an image, and lower the resolution. It is, therefore, preferable that the toner be deposited with a charge of 5 .mu.c/g to 10 .mu.c/g in mean value. In addition, the toner charge distribution should be stable, i.e., a minimum of toner particles of relatively low charge should be included in the toner which would lower the sharpness and resolution and contaminate the background.
As stated above, how to form two or more toner layers containing no uncharged particles and inversely charged particles and having a stable charge distribution of 5 .mu.c/g to 10 .mu.c/g on the developer carrier is the key to the equispeed development which increases the image forming speed and eliminates the local increase in image density.
Japanese Patent Application No. 15110/1990 corresponding to U.S. Ser. No. 07/597,881 filed Oct. 12, 1990 discloses a developing device including a developer carrier having fine dielectric portions and fine conductive portions distributed either regularly or irregularly on the surface thereof. The conductive portions are connected to ground. A developer supply member is rotatable at a position where it contacts the surface of the developer carrier. A single component type developer or toner is frictionally charged by the developer carrier and developer supply member. At the same time, the developer supply member and developer charge the dielectric portions by friction so as to form a great number of microfields in the vicinity of the surface of the developer carrier. As a result, the frictionally charged toner is retained on the developer carrier in multiple layers by the microfields. With this developing device, it is possible to form multiple toner layers having a stable charge distribution on the developer carrier. The present invention constitutes a further improvement over such a developing device.